Sony a7III PDAF striping – FAQs

Started May 2, 2018 | Discussions thread
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JimKasson
JimKasson Forum Pro • Posts: 30,794
Sony a7III PDAF striping – FAQs
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I'll still be contributing technical posts to the thread that I started on the PDAF striping on the a7III, but I wanted to back up from the quantitative minutia and give you all my take of the issue in general, approachable terms. I've created a little a7III PDAF striping FAQ, which is further down teh page, below a context-setting preambe.

Of course, i welcome comments and questions.

Those of you who are vaguely uncomfortable with numbers, get a cup of coffee and rejoice: this is the kind of post you'll like.

First off, I’m going to compare PDAF striping to several other Sony a7x problems:

  • Lossy raw compression
  • Shutter shock in the a7R
  • The ever-morphing “star-eater” digital filtering

All of these consumed a lot of Internet bandwidth in their day. None of those materially affected most people’s photographs most of the time. All of them affected some people’s images some of the time. All of them have partial or complete workarounds or mitigation protocols. Two of them have been fixed by Sony in subsequent cameras and/or firmware releases.

In my analysis, I‘m going to consider three things:

  • How likely is it that image damage is noticeable
  • How easy is it to know in advance whether the damage will likely occur
  • How objectionable is that damage
  • How onerous and effective are mitigation strategies.

Lossy raw compression is highly unlikely to materially damage images, the situations where such damage may occur are describable to and recognizable by a moderately-skilled photographer, the artifacts can be fairly ugly, and, unless you have a camera that lets you turn off the compression, mitigation can be moderately difficult.

A7R shutter shock is moderately likely to damage image, the situations leading to damage are moderately difficult to predict, the damage is usually not particularly objectionable (to the point where many think there is none), and highly effective mitigation is a real pain.

Star-eater filtering is no problem at all except for those photographers who engage in esoteric pursuits like making tracked night-sky images and maybe photographing fabrics with long exposures. It is easy to know if you’re likely to have a problem, the damage is only objectionable to a few, and one kind of mitigation is hard (the other, avoiding the problem shutter speeds, is an inconvenience to some, and too much trouble to consider to others).

PDAF striping is not likely to materially damage images. Predicting exactly when it will occur and how badly is difficult, but it’s pretty easy to describe a broad range on circumstances where it will never occur. The damage is at least as ugly as the raw compression artifacts. We already have some mitigation techniques, and I expect we’ll have more in the near future.

If I had to rank-order the above Sony vicissitudes in terms of their effect on most people most of the time, from worst to best, my list would be:

  1. A7R shutter shock
  2. Sony raw compression, PDAF striping (tie)
  3. Star-eater filtering

So, without further ado, as Jack Webb used to say, “Just the FAQs.” (I may have misremembered that line.)

Do other cameras have PDAF striping? They do indeed. In full-frame Sonyland, PDAF striping first appeared on the a7RIII. It also occurs on the a9, and now the a7III. Some other cameras show these kinds of artifacts.

Is it worse on the a7III? It’s about the same as on the a9, and both those cameras are worse than the a7RIII.

Those other two cameras have been out awhile. I never heard about this before. What’s the big deal with the a7III? Hard to say. I suspect that the a9, being optimized for sports and wildlife photography, is less likely to be used in the circumstances that provoke the striping. It may be that the a9, expensive specialist camera that it is, just shipped in far lower quantities than the a7III. Maybe the circumstances at Sony’s a7III launch event were especially conducive to triggering the striping.

Is it worse with some lenses? Indeed it is. The Sony 85 mm f/1.8 is especially problematic.

Are some lenses immune? If you really push things, probably not. I’ve not found a lens in which the right amount of flare in the right place pushed hard in postproduction won’t cause striping.

Are some lenses so good with respect to striping that I can not worry about it? Yes. I put the Sony 12-24/4 in that category, but it is possible to make it stripe on the a7III if you want it to, and I have seen an image in which it looks like the striping was caused accidentally.

Can I get striping without lens flare? I don’t think so.

If I see lens flare, does that mean that there’s striping? No.

Is there any place in the image where it’s especially problematical to put a bright light source? Technically, yes, but it varies with lens, and is probably not worth trying to track down.

Does increasing the ISO setting reduce the striping? Yes, it does. By ISO 3200 or so, it’s hardly ever noticeable, being masked by noise.

I just post images on the web. Do I have anything to worry about? No.

Is this something people will see looking at full frame images on a 4K screen? Unlikely.

How big do prints have to be before you see the artifacts? It depends on how bad it is, how good your vision is, and how close you get, but by and large you’re safe at 8x10 inches and smaller. At 17x22, you might take some care. Bigger than that, you should have a good look at images taken in problem situations before you spend a lot of money on a print.

How likely is an amateur who’s not paying attention to wander into a situation that causes striping? Not very likely. There are a few exceptions: sunsets, wide-angle shots with the sun in the frame, strong flare from stage lighting, and the like.

How likely is an amateur who’s not paying attention to see the striping if it’s there? Not very likely. See the discussion about prints and screen viewing above.

How likely is a striping-clueless pro advertising photographer to provoke the striping? Since PDAF striping came up, I’ve been looking at advertising images with an eye to answering that question. I see a lot of images where striping could be a problem. It seems that strong backlighting with the light sources visible in the frame, and pronounced lens flare is quite fashionable these days, and pros with deep pockets have no difficulty affording powerful strobes.

How likely is a pro advertising photographer to be using an a7III or an a9? Not very likely, especially now, unless they never employ the kind of lighting that causes the problem. I think that, if they’ve recently upgraded their gear, they’re more likely to be using a D850, an a7RIII, a GFX, or a high-end Canon. If they haven’t, they’re probably happy with their D3x.

How easy is it to fix the problem after the fact? If there’s not a lot of detail in the area with the striping, it’s simple to fix it in Photoshop. If there is, pretty good fixes are not hard. It is best fixed before or during raw development. Raw Therapee already has a fix in the current beta version. There’s another browser-based fix available on the web. I think more raw developers with be taking their cue from Raw Therapee.

If it’s so easy to fix, what’s the issue for the pros? They may not find the striping before the client sees it. It’s not all that easy to see sometimes, but once you’ve seen it, it’s distracting, artificial-looking, and frankly, ugly. Not something you want to have get in the client’s hands.

This is making my head hurt. Net it out for me. Did Sony make a good tradeoff when the increased the PDAF areas? I think they did. In the case of the a9, that is super important to their target customers, and the likelihood of striping for those folks is very low. In the case of the a7III, with an almost-entirely-amateur customer base (my guess), it makes really desirable features like eye-AF work much better, and the chances of that group of people simultaneously triggering the striping, noticing it, and finding it objectionable are low.

Maybe I should just buy a DSLR and forget it. If you are otherwise a candidate for the a7III, I think that would be throwing out the baby with the bathwater. There are a host of downsides to a DSLR (like lens AF tuning), as well as some upsides.

Jim

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